This invention relates to an electrochemical device for the measurement of oxygen pressures and more particularly concerns as electrochemical oxygen gauge of the type having an internal reference pressure and solid electrolyte.
The invention also concerns the application of such gauges to the measurement of partial pressures of oxygen, particularly in gazeous mixtures, and notably in combustion gases. It further concerns their application to the determination of the relative quantities of other gases in these mixtures, particularly with a view to the regulation of their composition.
The gauges of this type comprise an electrolyte in contact with two electrical conductors located respectively in different environments comprising oxygen.
These conductors are comparable in their operation to electrodes and form with the electrolyte a chain which can be considered as an electrochemical system.
One of the electrodes, the reference electrode, is situated in a compartment containing a chemical system which establishes within that compartment an oxygen pressure which acts as the reference pressure. This system, which will be designated hereinafter by the expression "internal reference", is generally made up either by a gaseous mixture such as an oxygen-gas, CO-CO.sub.2 or H.sub.2 -H.sub.2 O mixture, by pure oxygen, by a metal-metal oxide mixture or even by a mixture of two oxides of the same metal.
The reference electrode is in contact with an electrolyte made up by an ionic conductor capable of permitting the transfer only of oxygen ions. This electrolyte can be composed of a ceramic oxide or a vitreous phase having true ionic conduction, for example, it may be a solid solution of oxides depleted in oxide ions and having the fluorite structure, or a vitreous phase such as that described by D.YUAN and F. A. KROEGER in J. Electrochem. Soc. 118, 841 (1971).
The second electrode, the measurement electrode, is also in contact with the electrolyte but lies outside the reference compartment, and is placed in the oxygen-containing atmosphere that is desired to analyse. The electrode together with the atmosphere under test constitutes the measurement compartment.
The presence of oxygen in the oxide state an oxygen in the reduced state in contact with the electrodes of each of the reference and measurement compartments leads to a reaction at these electrodes, and thus to a potential difference being established between them. This potential difference obeys the Nernst equation, according to which: EQU E=RT/4F Ln P.sub.O.sbsb.2ref./ P.sub.O.sbsb.2mes.
In this formula, the various symbols used have the following meanings:
E=electromotive force in volts
R=ideal gas constant
T=temperature in degrees Kelvin
F=Faraday number
P.sub.O.sbsb.2 ref =oxygen pressure in the reference compartment
P.sub.O.sbsb.2 mes =oxygen pressure in the measurement compartment
If the reference oxygen pressure and the temperature are constant, the reading of the potential difference is a measure of the partial pressure of oxygen in the measurement compartment.
Furthermore, it is known that this partial pressure of oxygen in a gaseous mixture in an oxidation-reduction equilibrium is related by the law of mass action to partial pressures of other gaseous constituents of the mixture. Measurement of the partial pressure of oxygen as described hereinbefore can therefore provide a means of discovering the partial pressures of the other constituents of the mixture and by this means the composition of the mixture can be determined.
The significance of such an application of the gauge of the invention is evident, especially for the measurement of the concentration of the principal constituents of a combustion gas. It enables the composition of the mixture of combustive and combustible gaseous components (the combustive-combustible mixture) to be regulated so as to obtain better performance. In addition, it may be possible to effect regulation of the mixture so as to lower the amount of toxic gases formed from the combustion gas, and in particular to lower the level of carbon monoxide.
The partial pressure of oxygen in the system CO+1/2 O.sub.2 .revreaction.CO.sub.2 is related under conditions of thermodynamic equilibrium to that of carbon monoxide (CO) and carbon dioxide (CO.sub.2) by the law of mass action so that by adjusting the partial pressure of oxygen appropriately the partial pressure of CO can be lowered.
However, to perform regulations of this type it is necessary to have a gauge of high performance which in particular must be capable of giving very accurate measurements of the partial pressures of the constituents of the gas being analysed. Moreover, development of oxygen gauges on a commercial scale, especially when they are intended for the above applications, requires the gauges to be compact and capable of operation without needing additional equipment. However, until now gauges intended for the applications described above do not fulfil these needs in a satisfactory manner.
Generally the known gauges utilize air to establish a reference pressure, and to improve the accuracy of the measurements gas with a known partial pressure of oxygen is circulated within the reference compartment. These gauges are extremely inconvenient and do not give sufficiently precise results.
Other known gauges are those in which the internal reference is a metal-metal oxide mixture. Those gauges of this type which employ a palladium (Pd): palladium oxide (PdO) redox system, corresponding to the equilibrium Pd+1/2 O.sub.2 .revreaction.PdO, possess very interesting properties when compared with gauges containing other metal-metal oxide mixtures as the internal reference. In particular they enable accurate measurements of oxygen partial pressures to be obtained. However, even though the use of such a redox system gives the gauges some useful properties, it has not proved entirely suitable as an internal reference in the aforementioned applications, and it is particularly inconvenient for the determination of the relation between the partial pressures of CO and CO.sub.2 --that is to say P.sub.CO /P.sub.CO2,--in combustion gases. The temperature dependance of this redox system when employed as an internal reference is such that the voltages produced by gauges using the redox system vary considerably with temperature. Thus, measurements taken when the operating temperature is variable do not lead directly to the actual composition of the gas, and particularly do not provide the actual value of the ratio of the partial pressures of CO and CO.sub.2. In order to try to alleviate this problem and obtain constant voltage outputs despite possible variations in operating temperatures it is necessary to equip gauges employing this type of system with some means of temperature regulation. This makes the gauges unsuitable because of their increased size and the increased overall cost of the equipment.